It is well known that circulating fluidized bed combustion is performed under conditions that are relatively favorable to reducing emissions of nitrogen oxides (nitrogen monoxide and nitrogen dioxide) and emissions of sulfur oxides (sulfur dioxide and sulfur trioxide). Since the combustion temperature is about 850.degree. C. and the air intake is in stages, nitrogen oxides are produced in small quantities only, in general less than 200 mg/Nm.sup.3 relative to dry flue gases having a residual oxygen content of 6%.
Sulfur oxides are currently collected by inserting a solid desulfurizing agent such as a calcareous rock into the combustion furnace. It is possible to collect about 95% of the sulfur oxides because the solids are mixed thoroughly as a result of the internal and external re-circulation, and of the combustion temperature of about 850.degree. C.
However, any increase in desulfurizing performance levels by increasing the flow-rate of the desulfurizing agent would give rise to a substantial increase in nitrogen oxide emissions because of the catalytic effect of the desulfurizing agent on the conversion of nitrogen into nitrogen oxides. The known method is therefore not effective enough if its performance levels are to be improved, e.g. if the regulations become tighter.
Moreover, because of the low combustion temperature, the circulating fluidized bed gives rise to nitrous oxide (N.sub.2 O) emissions. Increasing the temperature so as to reduce nitrous oxide emissions gives rise to a detrimental reduction in desulfurization effectiveness.
Finally, if a surplus of desulfurizing agent is inserted into the furnace so as to obtain high desulfurization efficiency, then the circulating fluidized bed produces large quantities of solid residue.